Cardiovascular diseases (CVD), such as e.g. atherosclerosis, hypertension and ischemia, remain the leading cause of death in most developed countries as they cause permanent damage to the heart and blood vessels, which may lead to chronic heart failure, angina, or myocardial infarction (heart attack). For a patient showing symptoms of a cardiovascular disease, primary diagnosis and treatment are usually performed via interventional cardiology in a cardiac catheterization laboratory. Cardiac catheterization thereby means insertion of small tubes (catheters) through arteries and/or veins to the heart. In order to visualize coronary arteries and cardiac chambers with real-time X-ray imaging, a contrast agent is injected through the catheter. The contrast agent has to be opaque to X-rays and provide good image contrast as it flows into the coronary artery system or into the cardiac chambers. This procedure produces an image referred to as an angiogram, which is standard for diagnosing cardiovascular disease.
In the recent thirty years, X-ray guided interventional cardiology has grown considerably, fueled by demographic, technologic and economic factors. According to the American Heart Association (AHA), the number of interventional cardiology procedures grew by 470% in the United States between 1979 and 2003. New catheter-based interventional tools (such as e.g. balloon catheters and stents) allow physicians to treat more conditions and more complicated patient cases. As these new minimally invasive, image-guided procedures have positive patient outcomes and are less costly than open-heart procedures, physicians are actively encouraged by governmental and private payers to use these procedures for treating patients.
Nowadays, X-ray based cardiac catheterization systems represent the current standard of care and provide imaging modalities for both diagnostic and therapeutic procedures in cardiology. They are applied for generating real-time images of obstructions to blood flow in the coronary arteries. When an obstruction is identified, real-time X-ray imaging is utilized to guide insertion of balloon-tipped catheters to the point of obstruction for treatment by angioplasty (which means by balloon expansion of the restricted flow area in the artery) and stent placement (that is, by expanding a supporting structure to keep the newly enlarged artery open). The goal of therapy for patients with coronary artery disease is to alleviate symptoms of angina and reduce the risk of death or myocardial infarction by employing techniques and devices for re-opening the coronary arteries.
A cardiac catheterization system as mentioned above virtually enables all minimally invasive procedures in a catheterization laboratory. Currently developed systems all have the same fundamental architecture and use a point X-ray source that projects an X-ray beam through the patient and onto a large-area detector, the latter being used for converting the generated fluoroscopic image to electrical signals for display on a monitor. Thereby, a shadowgram image of the patient is obtained.
Conventionally employed cardiac catheterization systems typically perform two distinct types of real-time X-ray imaging: diagnostic angiography and interventional imaging. Diagnostic angiography is performed with a high radiation exposure in order to produce high-quality images. This diagnostic (cine) mode produces images of injected contrast agent flowing through the coronary arteries to diagnose the initial condition of the coronary arteries, determine the intervention required, and re-evaluate the coronary arteries after the intervention. Interventional imaging is performed with a regulated radiation exposure that produces lower-quality images. This interventional (fluoro) mode thereby provides real-time imaging of a patient's anatomy to guide the intervention and is used when inserting devices into the anatomy. The interventional mode is used for approximately 90% of the procedure imaging time.
While cardiovascular diseases primarily affect a patient's blood flow, cardiac electrophysiology (EP), a specific domain of interventional cardiology, involves the study of electrical abnormalities of the heart. Physicians use intra-cardiac catheters to locate and cure electrical dysfunctions of the patient's heart rhythm under X-ray fluoroscopy guidance. Congenital problems or diseased tissue in the heart can affect the electrical conduction leading to an irregular heart beat, including atrial fibrillation (AF). In this disease, the two upper chambers of the heart, the atria, do not beat efficiently, and blood is not pumped completely out of them, so it may pool and clot. If a part of the clot leaves the heart, it may cause a stroke or a pulmonary embolism. For the treatment of atrial fibrillation, certain areas of tissue may be ablated with radiofrequency energy so as to cure the anomalous electrical conduction and to permanently restore the normal heart rhythm. More precisely, the heart tissue is mapped to find the areas of abnormal electrical activity and ablated by cardiac electrophysiology to kill pathological tissue in certain areas. This procedure is commonly referred to as “mapping and zapping”. The procedures to locate and ablate the appropriate areas of tissue are extremely lengthy. A patient may spend between three and six hours in the cardiac catheterization laboratory, which may include up to 90 minutes of sheer imaging time. The patient receives significant amounts of X-rays up to an equivalent of 30,000 chest X-rays, and the electrophysiologist doing the procedures usually also receives a considerable dose of scattered radiation. Electrophysiology diagnosis and treatment does not require the injection of contrast agent into the coronary arteries to produce detailed angiograms and therefore requires somewhat lower imaging capability. The long procedure times place a high value on radiation exposure reduction.
Another important EP procedure is the placement of a pacemaker for a cardiac resynchronization therapy (CRT) during which a pacemaker lead has to be placed in a coronary vein. Electrophysiologists need a special training to perfectly know the anatomy and the access pathways to all the sites of interest and some practice to select the correct devices and manipulate them to target.
The patient's anatomy can be recorded with 3D imaging devices (CT, MRI) or by injecting contrast agent locally just at the beginning of the intervention (left atrium (LA) and ostium of the pulmonary veins (PV) for atrial fibrillation and coronary veins and sinus for CRT), but the physician has to perform mental registration to navigate in the live fluoroscopy images where this information is not visible anymore.
For AF procedures, knowing the exact positions of the catheters when measuring electrical potentials is key to find the sources that cause fibrillation (ectopic foci, re-entry loop). Even more important is anatomical mapping of the ablation sites in order to perform the desired ablation patterns, such as e.g. pulmonary vein isolation or roof line ablation in the left atrium.
Today, virtually all currently available conventional X-ray based cardiac catheterization systems, such as those developed and marketed by Philips Medical, Siemens Medical, GE Medical and Toshiba Medical, use the same fundamental imaging technology, that has not changed dramatically over the past 40 years. Incremental improvements to individual component have optimized system performance over decades to close to the theoretical limits. However, current systems still exhibit significant problems with poor image quality and high radiation exposure. The key problems thereby relate to imaging, radiation hazards and operational issues.
The most difficult imaging task in the cardiac catheterization lab is imaging large patients or imaging patients at steep viewing angles. With conventional systems, a large-area detector close to the patient causes more scattered radiation reaching the detector than image radiation, severely degrading image quality. Therefore, physicians often use the high-radiation diagnostic (cine) mode during interventions to obtain better quality images.
Moreover, best image quality is only possible for a short period of time. Conventional cardiac catheterization systems can only run in the diagnostic (cine) mode for approximately 20 seconds before the X-ray tube reaches its maximum temperature and shuts down automatically. It may take several minutes before the X-ray source cools down and imaging can resume.
In addition to that, overlying anatomy may inhibit viewing and navigation. Conventional cardiac catheterization systems produce a shadowgram image that shows objects with no depth information. Discerning 3-D anatomy from these flat images is difficult. In addition, image clutter and shadowing of the heart by ribs or the spine often degrades image clarity.
Another problem conventional X-ray based cardiac catheterization systems are typically faced with is exposing both the patient and the interventionalist to excessive radiation. Conventional systems expose patients to the equivalent of 200 to 500 chest X-rays per minute in the interventional (fluoro) mode. With up to 60 minutes of imaging time during a long interventional procedure, patients can be exposed to the equivalent of 12,000 to 30,000 chest X-rays per procedure. Such a prolonged exposure can cause radiation skin burns on patients and increase the risk of cancer to the interventionalists and catheterization lab staff. Radiation exposure risk is particularly acute in certain electrophysiology procedures due to long exposures of single areas of anatomy. Preventative measures for physicians include use of heavy and cumbersome wrap-around lead aprons and vests, thyroid shields, and goggles.
Furthermore, access to patient may be obstructed by the X-ray detector. Conventional cardiac catheterization systems require that the large-area detector is positioned close to the patient, thus restricting access to the patient by the clinical staff. This design is not only claustrophobic for the patient, but is also an obstruction if cardiac pulmonary resuscitation (CPR) is required.
As briefly mentioned above, electrophysiological procedures currently guided by fluoroscopy, and particularly atrial fibrillation, often take several hours. The main task of such procedures is to place catheters or cardiovascular stents at a given location in the interior of the myocard or in a cardiac blood vessel, respectively. This is usually done under guidance of intraoperative X-ray imaging in order to visualize the position of the catheter tip. Intraoperative application of fluoroscopic X-ray imaging is often necessary to provide answers for a large number of questions. This is especially true, for instance, if a surgeon needs to visualize the morphology of blood vessels. Apart from being applied in various surgical disciplines to assist in the placement of cardiac pacemakers, surgical stents and guide wires, this imaging modality is also used in orthopedic traumatology to enable the position monitoring of medical implants, orthopedic protheses as well as surgical screws and nails. In cardiac X-ray images, on the other hand, specific high-density anatomical structures (such as e.g. the spine, specific vertebras, etc.) or foreign objects (such as e.g. pacemaker leads and surgical stitches, etc.) are most of the time visible in the X-ray image and may thus at least partly obstruct or jeopardize the visibility, detection and/or tracking of interventional tools, either because they create similar patterns or because they cast a shadow on the objects which shall be detected. Classical image subtraction techniques do not help in case of slowly moving interventional tools and would require new acquisitions of reference sequences every time the 2D view changes.
US 2003/0 181 809 A1 describes a method for visualizing a medical instrument (such as e.g. a catheter that is used during a cardiological examination or treatment) which has been introduced into an area of examination within a patient's body. The herein disclosed method can be understood as an application of a cardiac roadmapping procedure, where a 3D reconstructed angiogram is used to add vessel information to an intraoperatively generated X-ray image. To be more precisely, said method comprises the steps of using a 3D image set of the area of examination and generating a 3D reconstructed image of this area, taking at least one 2D X-ray image of the area of examination in which the instrument is visualized, registering the 3D reconstructed image relative to the 2D X-ray image, visualizing the 3D reconstructed image and superimposing the 2D X-ray image over the 3D reconstructed image on a monitor.